1. Field of the Invention
The present invention relates to a heat-treatment apparatus for heat-treating a semiconductor part such as a semiconductor element or a semiconductor wafer in a predetermined gas atmosphere and, more particularly, to a vertical heat-treatment apparatus for vertically stacking semiconductor parts to be heat-treated and inserting the parts from the above into a furnace for performing a heat-treatment.
2. Description of the Related Art
A horizontal heat-treatment apparatus is known as an apparatus for heat-treating a plurality of semiconductor wafers. This heat-treatment apparatus is constituted by a heat-treatment tube horizontally set to receive a reaction gas for a heat-treatment therein. A plurality of vertically set semiconductor wafers are horizontally inserted in the heat-treatment tube, and the tube is heated by an externally set heater mechanism.
As semiconductor wafer manufacturing techniques have progressed, however, the size of a semiconductor wafer has been increased. When semiconductor wafers having such a large size are vertically set, it is difficult to control a reaction gas to uniformly act on the surfaces of the semiconductor wafers and to increase a space efficiency in a heat-treatment tube.
In a vertical heat-treatment apparatus, it is easy to control the reaction gas and to increase the space efficiency. Therefore, there is the trend that vertical heat-treatment apparatuses are used in increasing numbers.
A characteristic feature of such a vertical heattreatment apparatus is that a mechanism for rotating a boat having a plurality of semiconductor wafers or the like thereon and placed in the heat-treatment tube can be used. That is, in a vertical heat-treatment apparatus of, e.g., a down-load type, a rod is inserted from above into a central axis portion of a vertically set heat-treatment tube, and a boat having a plurality of semiconductor wafers thereon is suspended from the lower end portion of the rod, thereby placing the semiconductor wafers in the heat-treatment tube. In this case, a plurality of horizontally set semiconductor wafers are vertically stacked with predetermined intervals therebetween and can be rotated in the horizontal plane by rotating the rod.
In most vertical heat-treatment tube of this type, a heat-treatment gas is supplied from a lower portion of the tube and exhausted from the above the boat mounting semiconductor wafers. If the rod suspending the boat mounting the horizontally set semiconductor wafers is rotated in this state, the semiconductor wafers are also rotated in respective horizontal planes. By performing a heat-treatment in this state, therefore, uniform characteristics can be easily obtained on the entire surface of each semiconductor wafer.
In such a vertical heat-treatment apparatus, a detachable cap is attached to an opening portion formed in the upper portion of the heat-treatment tube, and an opening for inserting the rod is formed in the central portion of the cap. The rod is inserted into the heattreatment tube through the central opening portion of the cap and rotated together with the boat suspended therefrom. It is, therefore, difficult to provide a seal at the opening portion of the cap for inserting the rod.
Alternatively, a structure in which the cap and the rod are integrally formed may be adopted. When a mechanism for rotating the rod with such a structure is adopted, the rod cannot be rotated while the cap is in contact with the opening surface of the heat-treatment tube since the rod and the cap are vertically moved with each other. Therefore, the rod is moved to a position at which the cap is slightly separated from the opening surface of the heat-treatment tube, and is rotated while a small gap is formed between the cap and the opening surface of the heat-treatment tube. In such a structure, therefore, it is more difficult to form an airtight structure between the opening surface of the heat-treatment tube and the cap.
In a vertical heat-treatment apparatus of an upload type, unlike in the down-load type apparatus as described above, a boat having semiconductor wafers thereon is mounted on a heat-insulating quartz base, and this heat-insulating quartz base is vertically moved by a lift mechanism using a ball screw, a chain driving mechanism, or a hydraulic cylinder. The semiconductor wafers are inserted from a bottom opening portion of the vertically set heat-treatment tube into the heattreatment tube.
When semiconductor wafers are to be set in the heat-treatment tube in this manner, a flange portion of the heat-insulating quartz base and a flange portion around the opening portion of the heat-treatment tube are controlled by using a position sensor mechanism to form a small gap of about 1 mm therebetween when they are stopped. In this state, the boat is rotated together with the heat-insulating quartz base by a rotating mechanism to improve heat-treatment characteristics.
In the up-load type heat-treatment apparatus having the above arrangement, when quartz materials of the heat-insulating quartz base and the heat-treatment tube are in slidable contact with each other by rotation control of the boat, particles are produced by friction therebetween. In order to prevent production of particles, therefore, a gap of about 1 mm is formed between the heat-insulating quartz base and the heat-treatment tube. As a result, it is difficult to form an air-tight structure in this portion.
In order to perform a heat-treatment for semiconductor wafers, a heat-treatment gas is supplied into the heat-treatment tube. Since a combustible gas or phosphorus oxychloride (POCl.sub.3) or hydrogen chloride (HCl) gas which is harmful for a human body is used as the heat-treatment gas for semiconductor wafers, a care must be taken in handling of the gas or preventing a gas leakage to the outside. In addition, a care must be taken to prevent external air from flowing into the heat-treatment tube to degrade the internal atmosphere. For this reason, sealing structures between the heattreatment tube and the cap or the flange of the heatinsulating base and at the rod insertion portion are very important.
In such a vertical heat-treatment apparatus, the sealing portion is heated up to a temperature corresponding to the heat-treatment temperature, e.g., kept at a temperature as high as 200.degree. C. to 300.degree. C. In such a high-temperature state, the apparatus requires stable rotational motion of the rod, and needs to have a corrosion resistance with respect to the heat-treatment gas in contact, to be clean enough to prevent a metal gas or an organic gas from being mixed in the heattreatment gas, to remain free of particles properties. An example of the heat-treatment apparatus required to have the most strict characteristics is a diffusing system.
FIG. 4 shows a structure of a vertically arranged well-type heat-treatment apparatus to be used as, e.g., a diffusing system, which has a cylindrical heattreatment tube 51 with a bottom for forming a heattreatment furnace. The tube 51 consists of quartz and has an opening in its upper portion. A cap 52 consisting of quartz is placed on the opening surface of the tube 51 by its own weight. A surface of the tube 51 to be in contact with the cap 52 is formed into a ground-in surface to prevent an air leakage from between the opening surface of the tube 51 and the cap 52 when the cap 52 is placed on the opening surface of the tube 51. A chamber 53 consisting of quartz is formed integrally with the cap 52 so as to be fitted inside the tube 51. Quartz wool 54 is housed in the chamber 53 to form a heat-insulating member.
An opening 55 is formed in the central portion of the cap 52. The opening 55 is formed through the chamber 53 from the upper surface of the cap 52 so as to coincide with the central axis of the tube 51. The rod 56 is inserted into the tube 51 through the opening 55. A boat 58 supporting a plurality of semiconductor wafers 571, 572, . . . , is suspended from the lower end of the rod 56, and the semiconductors wafers 571, 572, . . . , to be heat-treated are housed in a predetermined position of the tube 51 by the rod 56.
The rod 56 is supported by a lift mechanism (not shown) and vertically moved as indicated by an arrow shown in FIG. 4. The rod 56 is also rotated by a mechanism including a motor. When the rod 56 is moved upward from the position shown in FIG. 4, the cap 52 is also moved upward by a flange 561 formed integrally with the rod 56, thereby extracting the boat 58 supporting semiconductor wafers outside the tube 51. In addition, when the rod 56 is moved downward after semiconductor wafers are set on the boat 58, the rod 56 is set at a predetermined position in the tube 51, and the cap 52 is set on the opening surface of the tube 51.
Although not shown, a gas for forming a heattreatment atmosphere is supplied from the bottom portion of the tube 51, and an exhaust duct 59 is formed in the upper portion (above the boat 58) of the tube 51. An amount of the gas supplied into the tube 51 is controlled, and the gas is forcibly exhausted from the duct 59. A pressure difference is set between the internal and external pressures of the tube 51 by adjusting the exhaust amount, and this pressure difference is minimized.
In the heat-treatment apparatus having the above arrangement, a gap is formed between the opening 55 of the cap 52 and the rod 56 so as not to prevent vertical and rotational movements of the rod 56. Therefore, the interior of the tube 51 communicates with the external air through the opening 55. Therefore, a gas leakage from the opening 55 must be reliably prevented.
A seal for preventing such a gas leakage is provided by a gas, and a gas supply duct 60 for this purpose is formed at a position close to the cap 52 of the tube 51. This duct 60 communicates with a ring-like chamber 61 formed around the upper opening portion of the tube 51, and the chamber 61 communicates with a path 63 constituted by a ring-like gap formed around the circumferential surface of the rod 56 through a path 62 formed in the inner surface of the cap 52. This path 63 communicates with the opening 55 of the cap 52 and is open to the interior of the tube 51. Although only one each of the ducts 59 and 60 is shown in FIG. 3, a plurality each of ducts 59 and ducts 60 are formed at equal intervals on the outer circumferential surface of the tube 51. In addition, a plurality of paths 62 are radially formed around the opening 55.
A sealing gas which is harmless for a human body, has no problem in safety, and has high purity not contaminating a to-be-heat-treated object (semiconductor wafer) such as nitrogen or oxygen is supplied from the gas supply duct 60 and partially discharged outside through the opening 55. At the same time, the gas is supplied into the tube 51 through the paths 62 and 63 and exhausted from the exhaust duct 59. A flow of the sealing gas, therefore, prevents a flow of a gas inside the tube 51 toward the opening 55, thereby reliably preventing a leakage of a heat-treatment gas to outside the tube 51.
In such a sealing structure, however, the sealing gas also flows into the tube 51 into which the heattreatment gas is supplied, thereby adversely affecting the heat-treatment atmosphere. That is, the concentration of the heat-treatment gas inside the tube 51 is decreased by the sealing gas. This dilution of the heat-treatment gas occurs more conspicuously in an upper portion of the tube 51. In addition, although the total amount of the sealing gas supplied from the duct 60 can be controlled, a flow amount of the gas into the tube 51 cannot be controlled. An active gas concentration and the like in the tube 51, therefore, is changed due to a variation in flow amount of the sealing gas into the tube 51, and important parameters such as a formation rate of an oxide film formed on the surface of a semiconductor wafer are varied accordingly. For example, when a pyrogenic oxidation process in which oxygen and hydrogen as materials are burned to produce water and an oxide film is formed by the water is to be performed, not only a growth rate of the oxide film is decreased, but also it is difficult to control the growth rate. In addition, since positions of a large number of semiconductor wafers in the tube 51 are different from each other, oxide films are undesirably grown at different growth rates.